在jdk1.5的时候,我们需要掌握枚举:反射、注解、泛型。现在java14都出来了
jdk1.8的新特性:函数式接口、链式编程、stream流、lambda表达式 都掌握的怎么样了?
本篇将着重说明 Stream 流的用法
面试题:
按条件筛选用户,请你只用一行代码完成!
- 1、id 为偶数
- 2、年龄大于24
- 3、用户名大写
- 4、用户名倒排序
- 5、输出一个用户
代码(User 实体类省略):
package stream;
import com.coding.test.entity.User;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class StreamDemo {
public static void main(String[] args) {
User user1 = new User(1,"jim",23,"北京");
User user2 = new User(2,"tom",24,"武汉");
User user3 = new User(3,"echo",25,"深圳");
User user4 = new User(4,"jerry",26,"上海");
User user5 = new User(5,"bob",27,"北京");
//数据库、集合 : 存数据的
// Stream:计算和处理数据交给 Stream
List<User> users = Arrays.asList(user1, user2, user3, user4, user5);
users.stream()
.filter(u->{return u.getId()%2 == 0;})
.filter(u->{return u.getAge() > 24;})
.map(u->{return u.getName().toUpperCase();})
.sorted((u1,u2)->{return u2.compareTo(u1);})
.limit(1)
.forEach(System.out::println);
}
}
接下来我们深入看看stream流中都有些什么?
创建流:
//1.创建一个具体字符串流
Stream<String> stream1 = Stream.of("A", "B", "C", "D");
//2.创建一个Stream流Builder<Object>对象
Stream.Builder<Object> builder = Stream.builder();
//3.创建一个空的String 流
Stream<String> empty = Stream.empty();
//4.合并两个流
Stream<String> concat = Stream.concat(stream1, empty);
//5.用迭代器创建无限流
Stream<Integer> iterate = Stream.iterate(1, x -> x + 1);
//6.生成 无限流
Stream<Double> generate = Stream.generate(() -> Math.random());
//7.collection的串行流 和并行流
List<String> list = Arrays.asList("1","2","3","4");
Stream<String> stream2 = list.stream();
Stream<String> stream3 = list.parallelStream();
//8.Arrays.stream创建一个数组流
IntStream stream = Arrays.stream(new int[]{1, 2, 3, 4});
//9.通过文件生成字符串流
Stream<String> stream = Files.lines(Paths.get("text.txt"), Charset.defaultCharset());
流的使用:
看的流的使用 也就是看users.stream()能点出来哪些东西,因为太多,这里就举例说明常用的几种:
1.filter过滤:
// 筛选出>3的数据
List<Integer> list = Arrays.asList(1,2,3,4);
list.stream().filter((i)->{return i > 3;}).forEach(System.out::println);
// 输出4
2.limit限流
// 获取未来7天的日期(顺便看看iterate 和 generate的用法)
Stream.iterate(LocalDate.now(), date -> date.plusDays(1)).limit(7).forEach(date-> {
System.out.print(date+",");
});
// 输出 2020-03-12,2020-03-13,2020-03-14,2020-03-15,2020-03-16,2020-03-17,2020-03-18,
AtomicInteger a = new AtomicInteger(0);
// 截取前三个随机数 并打印
// 写在这里的 时候就想换行输出下 加个计数器判断下
// 至于为什么用AtomicInteger计数而不是int
// 因为“ Variable used in lambda expression should be final or effectively final”
// AtomicInteger 在另一篇博客【并发编程之美-JUC]中有提到过
Stream.generate(()->Math.random()).limit(3).forEach(d->{
a.getAndIncrement();
if(a.get() > 1){
System.out.print(d+",");
}else{
System.out.print("\n"+d+",");
}
});
//输出0.8662508892898771,0.26661993344781665,0.2584450405261183,
3.skip 跳出
//skip(n)去掉前n个元素的流
List<String> list = Arrays.asList("1","2","3","4");
//若流中元素不足n个,则返回一个空,与limit(n)互补。
list.stream().skip(3).forEach(System.out::print);
//输出4
4.sorted排序
//倒序排列
List<String> list = Arrays.asList("1","3","5","2","4");
list.stream().sorted((o1, o2)->{return o2.compareTo(o1);}).forEach(System.out::print);
//输出54321
5.distinct筛选
//去除重复数据
List<String> list = Arrays.asList("1","2","3","1","2");
list.stream().distinct().forEach(System.out::print);
//输出123
6.映射
// 流式计算将实体中某两个属性对应组装成key value的格式返回
// 项目中一般读取数据字典 根据code 返回前台数据使用
Map<Integer, String> collect = users.stream()
.collect(Collectors.toMap(User::getId, User::getName));
System.out.println(collect.toString());
// 输出{1=jim, 2=tom, 3=echo, 4=jerry, 5=bob}
// 获取某个元素输出对应的List
List<Integer> ids = users.stream().map(user - >user .getId()).collect(Collectors.toList());
//将user按adress 分组
Map<String, List<User>> addressMap = users.stream()
.collect(Collectors.groupingBy(User::getAddress));
System.out.println(addressMap.toString());
//输出:{上海=[[email protected]], 武汉=[[email protected]],
//深圳=[[email protected]], 北京=[[email protected], [email protected]]}
小结:以上就是stream 流的常见用法 至于规约 查找 匹配都用的都很少,暂且不再深入 ,关于lambda表达式和函数式接口后面会写,还有一个很好玩的类Optional,后面也来写写看,奥利给!!!
原文地址:https://www.cnblogs.com/itiaotiao/p/12664908.html
时间: 2024-10-09 04:17:18